Mass spectrometer



1954 c. E. BERRY ETAL 2,688,088

MASS SPECTROMETER 8 Filed Oct. 19, 1951 2 Sheets-Sheet l F IG. 45

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sou/v00 ADJUSTABLE DC. SUPPLY OSCILLATOR m 40 69 INVENTORS- CLIFFORD E. BERRY BY CHARLES F. ROBINSON ATTORNEY Patented Aug. 31, 1954 UNITED "('1 ENT OFFICE MASS SPECTROMETER Clifford E. Berry, Altadena, and Charles F. Robinson, Pasadena, Calif, assignors to Consolidated Engineering Corporation,

Pasadena,

8 Claims.

This invention relates to mass spectrometry and particularly to methods and apparatus for analyzing gaseous mixtures in accordance with the ionic resonant frequency of the various components in the mixture.

In mass spectrometry, a gas sample is usually bombarded. moving electrons to produce ions of various substances present in the sample. The ions thus formed are separated into various components having different mass-to-charge ra tios by subjecting the ions to the influence of electric or magnetic fields or both.

In methods of mass spectrometry heretofore proposed, the individual components, separated as above, are caused to fall successively upon an ion collector by varying an electric or magnetic focusing field. The ions are discharged at the collector, and the intensity of the resulting current furnishes a measure of a partial pressure of the molecules (from which the particular ions were formed) in the sample. In one such method, ions are produced by the bombardment of the gaseous sample by an electron beam. The ions are caused to fiow in a semi-circular path through an evacuated envelope. Ions of different mass-tocharge ratio are caused to assume different radii of travel under the influence of a magnetic field, and each beam of ions of a given mass-to-charge ratio is successively focused on an ion collector located at an end of the semi-circular path.

The present invention involves a method of mass spectrometry based on the differences in fundamental resonant frequency of ions of different mass-to-charge ratio when subjected to a special type of electric field. This method of mass spectrometry possesses a number of advantages over the conventional methods particularly with regard to simplicity, compactness and versatility of the apparatus in which it may be carried out.

The present invention, therefore, contemplates in a method of mass spectrometry involving the formation of ions of a gaseous sample, the improvement which comprises subjecting the ions to a shaped D. C. field of such nature that the ions are caused to oscillate in the field at a frequency characteristic of ion mass, and measuring the effect of those ions oscillating at a particular frequency. The D. C. field is preferably made as nearly parabolic as practical. A comparatively weal: radio frequency A. C. field may be superimposed on the shaped D. C. field in the region of oscillation. For a given frequency F of such a superimposed field, ions of differing mass-to-charge ratio may be successively brought into resonance by varying the strength of the shaped D. C. field.

Essentially the method comprises shooting ions into the shaped D. C. field, which may or may not have an R. F. field superimposed thereon, from a position near a potential maximum, and allowing them to transfer energy to a tuned circuit as they oscillate in the field. Application of the R. F. field increases the sensitivity of the tuned circuit in a manner analogous to the operation of a regenerative radio receiver. The R. F. component may be derived froman oscillator and the ions of a given resonant frequency may be sensed by measuring the loading of the oscillator working at or near this frequency.

The invention is also directed to apparatus for carrying out the new method of mass spectrometry. In one embodiment, the apparatus comprises the combination of an analyzer chamber, means for producing a shaped D. C. field in the chamber, means for introducing ions into the shaped field at a point near a potential maximum, and means for measuring the inductive effect of the ions oscillating in the space at a given frequency.

In the preferred embodiment, the apparatus as described includes means, such as a tuned oscillator, for superimposing on the shaped D. C. field an R. F. field. As mentioned above, the particular ions in resonance with the frequency of such a superimposed A. C. field may be sensed by measuring the loading of the oscillator induced by resonant oscillation of these particular ions. Ions of differing mass-to-charge ratio may be brought into resonance with the R. F. field either by varying the frequency thereof or by varying the strength of the D. C. field.

A feature of the invention is the provision of means for eliminating harmonic interference. Ions of a mass-to-charge ratio differing from the mass-to-charge ratio of a given ion by factors of a, 9, 16, etc. tend to interfere in the process. One means of avoiding harmonic interference is by a preliminary gross separation of ions of harmonic resonant frequency. This is accomplished in one form of apparatus by subjecting the ions to a transverse magnetic field prior to introduction into the space. The magnetic field strength is adjusted so as to deflect ions to be measured from ions differing in mass-to-charge ratio from the given ions by a factor of 4 or more.

In one form of the apparatus of the invention, a radio frequency oscillator is coupled to a D. C. power supply circuit either by inductive coupling or by direct coupling to superimpose a radio frequency A. C. field on the D. C. field. The electric field in which the ions are caused to oscillate may be established by two connected electrodes disposed on opposite sides of the space and an intermediate oppositely charged electrode. Preferably the field is shaped by including a series of spaced grids, the objective being to establish, as nearly as possible, a parabolic potential distribution. Bunched oscillation of resonant ions in the space induces a counter E. M. F. in the intermediate electrode, which varies the load on the oscillator through the coupling circuit. Detection of resonance therefore can be accomplished by a plate current meter in the oscillator or by means of a milliammeter in the oscillator output circuit.

The invention may be more clearly understood from the following detailed description thereof taken in relation to the accompanying drawings in which:

Fig. 1 is a sectional elevation of a mass spectrometer according to the invention;

'ig. 2 is a sectional elevation of another type of mass spectrometer adapted to accomplish the method of the invention;

Fig. 3 is a graphic portrayal of ion travel and wave form of the induced oscillating current;

Fig. 4 is a partial sectional elevation of another embodiment of the invention characterized by a difference in induced signal; and

Fig. 5 is a partial sectional elevation of an embodiment of the invention including means for establishing an axial magnetic field in the analyzer chamber.

Referring to Fig. 1 of the drawing it will be observed that the mass spectrometer has an envelope 19 provided with an evacuating line I2, an ion source [3 and a sample inlet line [4. The ion source shown in the instruments of each of Figs. 1 and 2 is illustrated schematically only. Any conventional ion source employing an electron beam or other means for ionizing a gaseous sample may be employed.

The ion source I3 is provided with a propelling electrode it by means of which the ions are propelled toward the opposite end of the chamber It A series of electrodes are disposed in the chamber spaced from the ion source. Outer electrodes I8, USA are connected to one side of an adjustable D. C. power supply 22, and the central electrode i9 is connected to the opposite side of the D. C. supply. Intermediate electrodes 20 are connected through suitable resistors 2| intermediate the electrodes l8 and I9 and in such fashion as to produce a parabolic D. C. potential field in the region between the outer electrodes l8, [8A. All of the electrodes, with the exception of the end electrode [8A, must be perforated to permit oscillation of ions in the field developed thereby. The envelope It is conveniently coupled to one side of the D. C. source (lead 23) to improve the stability of the instrument.

A radio frequency oscillator 21% is transformer coupled to the power supply circuit through an air core transformer 26. Blocking condensers C1 and C2 serve to keep the D. C. voltage out of the oscillator circuits and a radio frequency choke 28 keeps A. C. voltage out of the power supply circuit. The output circuit of the oscillator 24 is provided with a series coupled A. C. milliammeter 3B.

As an alternative to the use of a separate ion source, ions may be formed within the grid system itself by an electron beam passing transverse to the axis of the chamber near a potential maximum point, for example in the region of electrode [8. Such a beam may be collimated by a weak parallel magnetic field, this being an expedient well known in the art. Such an arrangement has the advantage of simplicity in the elimination of a separately enclosed ion source, but for reasons of avoidance of spurious measurements or selective removal of harmonic ions, the illustrated construction is preferred.

In operation, a gas sample is introduced to the ion source I3 through the inlet tube M. In the ion source the sample molecules are ionized by means familiar in the art, as for example an electron beam, and the ions are propelled into the resolving field by means of a small potential difference established between the electrode I6 and the electrode [8. The positive ions, propelled from the ion source, pass into the space defined by electrodes I8 and I8A. Thereafter the potential difference between the electrodes l8 and I9 propels the ions toward the electrode 18A. After passing through the screen 19 the potential difference between the electrodes IBA and [9 causes the ions to reverse their direction of travel and return toward the electrode l8.

Ions which fail to lose energy to the oscillator during their first traversal of the system will continue through to strike and discharge on end plate ISA. Thus the only ions which remain after the first transit of the grid system are those whose entry into the system was at such a phase angle as to insure that they will actually transfer energy to the external circuit. The device is therefore self-punching, thereby insuring that the ion beam will actually induce a true periodic signal, and not merely a random noise.

The ions in the space between the electrodes I8, lBA will have a fundamental natural frequency of f cycles/sec. as given by the equation:

1 E Z m m tz where d=the distance in cm. between the electrodes l8 and I9 and I9 and [8A,

E=the D. C. voltage in e. s. u. impressed across the electrodes [8, [BA and I9.

e=the charge in e. s. u. of the given ions in the space, and

m=the mass of the ion in grams.

From the above equation it is clear that for an A. C. driving frequency F, E may be varied so as to bring ions with various charge-to-mass ratio (e/m) into resonance with F. By incorporation of a suitable meter in the alternating supply line the decreased impedance of the circuit as a result of resonant oscillation of bunched ions in the space between the electrodes, can be measured.

Equation 1 covers the case in which the electric fields on either side of the center electrode H! are uniform. The frequency of oscillation of the ions under these conditions is a function of oscillation amplitude, so that an ion will fall out of resonance as soon as it delivers enough energy to the detectin circuit to reduce its oscillation amplitude appreciably. It can be demonstrated mathematically that the ionic oscillation frequency is strictly independent of amplitude if the electric field is in the form represented by the expression:

where .r'=distance of ion from the axis of symmetry, and k is a constant of the system. In this situation the resonant frequency (f) of an ion is:

where m is the ion mass and q is the charge on the ion. Where frequency is independent of amplitude, the ion can deliver energy to the oscillating circuit until the ions kinetic energy is entirely dissipated, thus increasing sensitivity and improving the sharpness of response.

An alternative expression for the resonant frequency under conditions defined in Equation 2 where d=spacing from the axis of symmetry to the outermost grid, in cm. and V voltage between axis of symmetry and outermost rid (stat-volt) so that for a fixed oscillation frequency one ion or another can be brought to resonance by the proper choice of V. For example,

1f the overall length of the grid system is cm., the voltage 200 volts, a singly charged ion of mass 20 will have a natural frequency of 5.63 megacycles, whereas an ion of mass l9 will resonate at 5.78 megacycles. Two such signals are easily separated by conventional resonant circuits. Mass I9 is brought into resonance at a frequency of 5.63 megacycles with an applied potential (V) of 190 volts, so that adequate voltage resolution is obtained.

Although the use of a parabolic potential distribution as given in (2) will yield strictly sinusoidal motion of the ions, it will not necessarily result that the A. C. signal transmitted to the sensing means will be sinusoidal since the character of this signal will change according to the details of the mannerin which the ion beam is coupled to the oscillating circuit. Consequently, if the instrument is set to detect ions of mass mo, and resonant frequency In, and any ions of mass 4 me are present, thesignal from such ions has not only a fundamental frequency of io/Z but usually a harmonic component of in. This harmonic component will of course interfere with the analysis of the ions having a fundamental resonant frequency of in. This difficulty can be alleviated by inclusion of small trimmer condensers across voltage divider resistor 2| and can be completely eliminated by employing a preliminary coarse magnetic separation capable of selectin from a heterogeneous group of ions a band spanning a mass ratio of less than four to one.

Apparatus is shown in Fig. 2 for accomplishing such gross magnetic separation, the apparatus also including additional alternative features.

Referring to Fig. 2 the mass spectrometer is shown to include an analyzer chamber 40, in one end of which is disposed an ion source 4| having a sample inlet line 42. An evacuating line 43 connects the analyzer chamber to a suitable evacuation system not shown. The ion source 4! is provided with a propelling electrode 44. The right hand end of the chamber (as viewed in Fig. 2) is divided by a series of spaced electrodes. The outer electrodes 46 and 46A are connected to one side of an adjustable D. C. power supply 50. The center electrode 4! is connected to the opposite side of the power supply 50 and intermediate electrodes 48 are connected through suitable resistor 49 to shape the field as heretofore described. To permit entry of the ions between the electrodes into the field developed by the several electrodes and to permit oscillation of the ions therein, the electrodes with the excep tion of electrode 48A are perforated on the axis of the ion source M.

A radio frequency oscillator 52 is coupled to the electrode circuit through blocking condensers C3 and C4. A n ill-iammeter 55 rovides means for measuring the loading due to bunched oscillation of resonant ions.

A permanent magnet 58 establishes a magnetic field across the space between the ion source and the first electrode to provide the gross separa tion between ions difiering in mass-to charge ratio by a factor of 4 or more as described above. There is no attempt, by means of the magnetic field, to separate ions of approximately equal mass-to-charge ratio. The magnetic field deflects ions in a given mass range through the electrode it. Ions outside this mass range strike the electrode and are excluded from the space in which oscillation is brought about.

Considering Figs. 1 and 2, it is apparent that approximately uniform fields exist in each half of the space between the outer electrodes. An ion released with zero velocity at the left hand end of the chamber will be subject to a constant acceleration up to a velocity corresponding to the voltage of the center electrode. Thereafter the ion will be subjectto a constant deceleration to zero velocity, after which the motion repeats in the opposite direction. In Fig. 3, the ion velocity is plotted against time, the result being the saw tooth curve A. As plotted, the portion of the curve lying above the base line trepresents ion travel from left to right, while that lying below the base line represents that from right to left.

As far as the induced current on the center electrode is concerned, it makes no difference whether the ion is moving from left to right or right to left. Thus the frequency of the induced current is just twice the frequency corresponding to one cycle of ion motion. For the same season the driving frequency F must be twice the resonant frequency of the oscillating ions. Thus The instantaneous current- 2' induced in the external circuit by an oscillating ion is then given by the following equation:'

i=eEw (6) where e=the ion charge,

v=the instantaneous velocity, and

Ev=the field in the direction of motion that would exist at the particle if all the electrodes but the one in question were grounded, the particle removed, and the electrode in question raised to a unit potential.

In the simplest embodiment of the invention,

E1) is constant throughout the space, and o increases uniformly with time. Therefore with a single oscillating ion, the induced current in the external circuit is a saw tooth wave illustrated as curve B in Fig. 3. The induced current will have a frequency of twice that of the ions thus matching the driving frequency. The curve B in Fig.- 3 shows the wave form of the current induced by a single ion-.- For a bunch of ions occupying a finite length in the direction of motion, the wave would still have straight sides but the peaks would be more or less rounded depending upon the distribution of charge in the bunch.

Fig. 4 shows an alternative connection of several field shaping grids, each of the several grids being connected separately through a first voltage divider 60 to an oscillator circuit '62 and through a second voltage divider 64 to a D. C. source 65. In this arrangement the induced R. F. signal frequency and the ionic oscillation frequency are the same.

The advantage of the screen electrodes il1us-' trated in Fig. 1 over the perforated electrodes illustrated in Fig. 2 is in a freer ion flow in the field. Using perforated electrodes some diificulty may be encountered in keeping the oscillating ions on the axis of the apertures of the electrodes. Thus, to load the oscillator circuit sufficiently for analytical purposes it is necessary that a large number of ions be in resonant frequency at a given time. For this reason it is important that the resonant ions are kept on the axis of the apertures in the electrodes. Axial alignment of the resonant ions can be insured by the application of an axial magnetic field. Such an axial magnetic field may be established by the means shown in Fig. 5. This figure shows an instrument substantially identical to that of Fig. 2 with the additional provision of a co-axial solenoid 68 enclosing the envelope. In addition the envelope is grounded at 69 which difiers in construction but not in effect from the arrangement of Fig. 2.

The use of such a field will increase the efficiency and sensitivity of the device and is therefore a preferred but not a necessary feature.

Build up of an undesirable space charge is inherently avoided by continuous discharge of a fraction of the total number of ions on the several grids. The probability of collision with the grids is, of course, a function of the number of ions in the space so that an equilibrium value is realized and space charge is no problem.

Furthermore, it can be shown that when the ions have delivered most of their energy to the oscillator circuit, they are inherently attracted to grid I9 or plate 4'! and discharged thereon so that space charge due to low energy ions is automatically dissipated.

We claim:

1. In a mass spectrometer, the combination which comprises an analyzer chamber, a plurality of electrodes disposed in the chamber parallel to each other, means including said electrodes for developing a substantially parabolic D. C. field across a space in the chamber, a tuned circuit connected across said electrodes, means for introducing ions into the space and means for measuring the inductive efi'ect on the tuned circuit produced by ions oscillating under the infiuence of the D. C. field.

2. In a mass spectrometer, the combination which comprises a chamber, an ion source in the chamber, means for producing a substantially parabolic D. C. field across a space in the chamber, means for impressing an oscillating field across the space, means for propelling ions from the ion source into the space whereby ions of a given specific mass will oscillate in the space in resonance with the oscillating field, and means for measuring the inductive effect of the oscillating ions.

3. In a mass spectrometer, the combination which comprises a chamber, an ion source in the chamber, means for producing a substantially parabolic D. C. field across a space in the chamber, means for impressing an oscillating field across the space, means for propelling ions from the ion source toward the space, means for establishing a magnetic field in the chamber between the ion source and the space to deflect ions in a given mass range so that only a part of the ions will enter the space, and means for measuring the inductive effect of ion oscillation in the space.

4. In a mass spectrometer, the combination which comprises a chamber, an ion source in the chamber, means for producing a substantially parabolic D. C. field across a space in the chamber, means for varying the strength of the D. C. field, means for impressing an oscillating field across the space, means for propelling ions from the ion source toward the space, means for establishing a magnetic field in the chamber between the ion source and the space to deflect ions in a given mass range so that only a part of the ions will enter the space, and means for measuring the inductive efiect of ion oscillation in the space.

5. In a mass spectrometer, the combination which comprises a chamber, an ion source in the chamber, first and second electrodes on opposite sides of a space in the chamber, remote from the ion source, a third electrode intermediate the first and second electrodes, a source of variable D. C. voltage connected across the first and second electrodes and the third electrode, a radio frequency oscillator coupled across the electrodes for impressing an alternating field across the space, means for propelling ions from the source into the space whereby ions of a given specific mass will oscillate in the space in resonance with the alternating field, and means for measuring the inductive effect of the ions on the alternating field.

6. Apparatus according to claim 5 wherein the electrode nearest the ion source and the intermediate electrode are screen electrodes.

7. Apparatus according to claim 5 wherein the electrode nearest the ion source and the intermediate electrode are plate electrodes apertured to permit ion passage therethrough.

8. In a mass spectrometer, the combination which comprises a chamber, an ion source in the chamber, first and second electrodes on opposite sides of a space in the chamber remote from the ion source, a third electrode intermediate the first and second electrodes, a source of variable D. C. voltage connected across the first and second electrodes and the third electrode, a radio frequency oscillator coupled across the electrodes for impressing an alternating field across the space, means for propelling ions from the source into the space whereby ions of a given specific mass will oscillate in the space in resonance with the oscillator, means for producing a magnetic field between the ion source and the space to deflect part of the ions so that only the ions of a given mass range will enter the space, and means for measuring the inductive effect of ions oscillating in the space.

Name Date Schissel Oct. 2, 1951 Number 

